This invention relates generally to motor controls and more specifically to a control system for a switched reluctance motor.
This application is related to B. K. Bose et al. application Ser. No. 915,291, filed Oct. 3, 1986 and assigned to the instant assignee.
Although they have been known for some time, interest in switched reluctance motor (SRM) drives has recently revived. Compared to conventional induction and synchronous motor drive systems, the SRM drive is simple in construction and economical. In addition, the converter which supplies power to the SRM machine requires fewer power devices and, therefore, is more economical and reliable. In view of these advantages, the switched reluctance motor drive system provides an attractive alternative to conventional drive systems and is expected to find wide popularity in industrial applications.
Switched reluctance motors conventionally have multiple poles or teeth on both the stator and rotor (i.e. doubly salient). These are phase windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator poles is connected in series to form one phase of the multiphase switched reluctance motor.
Torque is produced by switching current on in each phase winding in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow so that unidirectional current pulses synchronized with rotor movement can be applied by a converter using unidirectional current switching elements such as thyristors or transistors.
In operation, each time a phase of the switched reluctance motor is switched on by closing a switch in a converter, current flows in the stator winding of that phase, providing energy from a DC supply to the motor. The energy drawn from the supply is converted partly into mechanical energy by causing the rotor to rotate toward a minimum reluctance configuration and partly into stored energy associated with the magnetic field. After the switch is opened, part of the stored magnetic energy is converted to mechanical output and part of the energy is returned to the DC source.
Most of the published literature relating to SRM drives concentrates on analysis of the machine and the configuration of the power converters; very few papers discuss the control aspects. The control requirements of the SRM drive are so unique that the concepts of induction and synchronous-type machines can hardly be extrapolated to the SRM. The SRM drives discussed in the literature are mainly open-loop control with angle and current amplitude regulation by manual adjustment, and have usually been designed with discrete components and dedicated hardware. Such prior control systems are frequently bulky, complex, expensive, limited in mode of operation, and hardware intensive. Although suitable for laboratory tests, an SRM with such a control system does not readily lend itself to industiral applications.
A need thus exists for a control system for a switched reluctance motor which overcomes the drawbacks of presentday designs and facilitates use of the switched reluctance motor for general purpose industrial applications.